The present invention relates to the conversion of plastics into finished articles and, more particularly, to the conversion of polybut-1-ene in the solid state.
It is known that the press-forging of certain thermoplastics enables finished articles having a large thickness or complicated shape to be manufactured cheaply, and that such articles cannot be satisfactorily manuactured by injection-molding. Thus, the press-forging of polypropylene and polyethylene is described in the following publications:
Society of Plastics Engineers, 26th Annual Technical Conference, New York, May 1968, pages 225-230;
Plastics and Polymers, Volume 38, April 1970, pages 124-130;
Machine Design, May 3, 1973, pages 94-99.
According to these publications, the press-forging of polyethylene and polypropylene is effected by heating the thermoplastic to a few degrees Centigrade below its melting point and subjecting it to a pressure of from 70 to 775 kgf/cm.sup.2 for a period of from 15 to 60 seconds.
No satisfactory method is known at the present time for converting polybut-1-ene into finished articles having a large thickness or a complicated shape. We have therefore investigated the possibility of manufacturing such articles by the press-forging of polybut-1-ene and have found that it is not possible to do this by simply using the conditions and techniques already disclosed for the press-forging of polyethylene and polypropylene. Polybut-1-ene is distinguished from these thermoplastics by the existence of a particularly complex and pressure-dependent polymorphism phenomenon, as described in J. Macromol. Sci. (Phys.), B1(2), pages 309-334 (1967). This phenomenon essentially manifests itself in the existence of a metastable crystalline form, referred to as the II form, which melts at 114.degree. C., and a stable crystalline form, referred to as the I form, which melts at 130.degree. C. Because of this phenomenon, lengthy experimentation has been required in order to determine the optimum conditions, from the industrial point of view, for the press-forging of polybut-1-ene.